This invention relates to a method for manufacturing plastic bags in a continuous way by operating a sealing member upon guided tubular foil of thermoplastic material, whereupon the bag formed is either separated from the foil band or perforations are made in the vicinity of a sealed joint for separating a bag.
Such a method is known per se. This method has, however, the drawback that owing to operation of the sealing member the plastic shrinks at the location of the sealed joint, which impairs the plastic foil and often will result in a sealed joint of poor quality.
It is evident that the strength of the bag decreases, as owing to shrinkage a weakening of the wall thickness is produced whereby the formation of cracks easily occurs when the bag is filled with material.
It has been attempted to obviate these difficulties by severing part of the tubular foil prior to formation of the sealed joint and to make the sealed joint at the free end of part of the tubular foil. This method has the advantage that the plastic can shrink along from both ends in the direction of the sealed joint whereby a weakening of the wall does not take place and the quality of the bag increases considerably.
This method for manufacturing bags has, however, the disadvantage that the bags cannot be put on the market as bands, comprising a row of bags, connected by means of perforations. At present such bands are preferred for delivering thin-walled plastic bags for packing household refuse.